The use of electronic article surveillance or security systems for detecting and preventing theft or unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, such systems, sometimes called EAS systems, employ a label or security tag, also known as an EAS tag, that is affixed to, associated with, or otherwise secured to an article or item to be protected or its packaging. Security tags may take on many different sizes, shapes, and forms, depending on the particular type of security system in use, the type and size of the article, etc. In general, such security systems are employed for detecting the presence or absence of an active security tag as the security tag and the protected article to which it is affixed pass through a security or surveillance zone or pass by or near a security checkpoint or surveillance station.
The security tags that are the subject of this invention are designed to work with electronic security systems that sense disturbances in radio frequency (RF) electromagnetic fields. Such electronic security systems generally establish an electromagnetic field in a controlled area defined by portals through which articles must pass in leaving the controlled premises. A resonant tag circuit is attached to each article, and the presence of the tag circuit in the controlled area is sensed by a receiving system to denote the unauthorized removal of an article. The tag circuit is deactivated, detuned or removed by authorized personnel from any article authorized to leave the premises to permit passage of the article through the controlled area with alarm activation. Most of the tags that operate on this principle are single-use, i.e., disposable tags, and are therefore designed to be produced at low cost in very large volumes.
In conventional practice, the inductor and capacitor elements that comprise the resonant circuit are fabricated by etching both sides of a substrate that consists of a 1 mil thick layer of polyethylene sandwiched between two layers of aluminum foil.
FIG. 1A is a scaled illustration of one side of a typical 1.5″ square RF tag showing a nine turn inductor coil on a 40 mil pitch, i.e., with 25 mil wide conductors separated by 15 mil spaces. FIG. 1A also depicts a triangular-shaped interconnection land area in one corner, and, positioned in the open space at the center of the coil, a capacitor plate. FIG. 1B illustrates the second side patterned with a matching capacitor plate in the center and a connecting link to the land area in the corner of the tag where a mechanical connection, typically formed by crimping or staking, joins the circuit patterns on side one to those on side two. Alternatively, the capacitor plate can be located outside the inductor coil in a corner of the tag but this configuration requires that the inductor pattern assume a roughly triangular shape. However, because planar inductor performance is optimized by placing as many coil turns as possible near the periphery of a square pattern, both of these design approaches are compromised by the need to devote tag surface area to the capacitor and interconnect functions.
Deactivation of these tags by direct means is problematic. Physical removal of tags that are adhesively or mechanically affixed to the protected article can be difficult and time consuming. Detuning the security tag by covering it with a special shielding device such as a metalized sticker is also time consuming and inefficient. Furthermore, both of these deactivation methods require the security tag to be identifiable and accessible, which prohibits the use of tags embedded within merchandise at undisclosed locations or tags concealed in or upon the packaging.
Improved deactivation methods incorporate remote electronic deactivation of a resonant tag circuit such that the deactivated tag can remain on an article properly leaving the premises. An example of such a deactivation system is described in U.S. Pat. No. 4,728,938 (Kaltner, 3/1988). Electronic deactivation of a resonant security tag involves changing or destroying the detection frequency resonance so that the security tag is no longer detected as an active security tag by the security system. There are many methods available for achieving electronic deactivation. In general, however, the known methods involve either short circuiting a portion of the resonant circuit or creating an open circuit within some portion of the resonant circuit to either spoil the Q of the circuit or shift the resonant frequency out of the frequency range of the detection system, or both.
A method of deactivating a tag by short circuiting a portion of its resonant circuit is disclosed in U.S. Pat. No. 4,498,076 (Lichtblau, 2/1985) entitled “Resonant Tag and Deactivator for Use in Electronic Security system” and U.S. Pat. No. 4,567,473 (Lichtblau, 1/1986) entitled “Resonant Tag and Deactivator for Use in Electronic Security System”. In this approach an indentation or dimple is made within the plates that form the capacitor portion of the resonant circuit. At energy levels higher than the detecting signal but within FCC regulations the deactivation device induces a voltage in the resonant circuit of the tag sufficient to cause the dielectric layer between the plates to break down in the area where the indentation has reduced the thickness of the dielectric layer. This type of security tag can be conveniently deactivated at a checkout counter or other such location by being momentarily placed above or near the deactivation device.
However, tags made by this method, which requires the precise formation of an approximately 0.1 mil indented thickness in a polymer layer that is typically only 1 mil thick to begin with, may not always function as designed. For example, if the indentation is not deep enough, i.e., if the polymer dielectric layer under the indentation is thicker than intended, the energy provided by the deactivating device may not be sufficient to cause breakdown of the dielectric layer. In retail establishments, this circumstance can lead to an embarrassing confrontation of innocent customers by store security personnel. On the other hand, if the indentation is too deep, i.e., the polymer dielectric layer under the indentation is thinner than intended, the tag may be prematurely deactivated by exposure to the lower energy detection signal emanating from the portals or the static charge that can build up on the packaging machinery used to automatically apply tags configured as product identification or pricing labels. In this case, retailers are not getting the protection they are paying their packaging suppliers to provide. Thus, with respect to the deactivation reliability of conventional EAS RF tags, no completely satisfactory method has emerged nor has the prior art taken the specific form of the novel approach proposed in this invention.
Retailers who employ anti-pilferage systems based on RF technology would like the tags that are used in these systems to be smaller in size, preferably 1″ square, so that they can be more easily concealed on or in the protected merchandise. They also perceive that smaller tags would consume less material and therefore cost less to produce. FIG. 2 is a scaled illustration of a 1″ square tag patterned with a nine turn 40 mil pitch inductor coil per FIG. 1A. However, as the overlay in this illustration reveals, it is impossible to repackage the coil geometry shown in FIG. 1A into a 1″ square format using the same conductor pitch. This is due to the fact that the resonant frequency of a tag circuit is defined as: F=½π√LC. Consequently, if L is reduced, C must increase by an offsetting amount if the resonant frequency of the circuit is to remain unchanged. In this example, conversion from a 1.5″ to 1.0″ format will reduce L by a factor of roughly 2, so C must increase by the same factor. C is defined as: C=k*A/t, where k is the dielectric constant of the polymer material, t is the thickness of the polymer layer, and A is the area of the capacitor plate. K is of course fixed by the choice of polymer material but in conventional practice t is effectively fixed at 1 mil by the laminating methods used to fabricate the substrate material. Plate area is therefore the only variable available to increase C. However, as illustrated by the overlay in this Figure, doubling the plate area of the capacitor in a 1″ square tag format requires the elimination of several coil turns, which significantly reduces the effective operating range of the tag circuit. Furthermore, as indicated by the overlay in the upper left-hand corner, the number of turns and shape of the coil pattern in a 1″ square format will also be adversely affected by the surface area that must be devoted to the mechanical interconnection. These consequences undoubtedly explain why the production of conventional RF tags remains standardized on a nominal 1.5″ square design format despite the fact that tags of this kind were developed and introduced to the market more than twenty years ago.
Thus, with respect to size as well as deactivation reliability, no completely satisfactory RF tag design for electronic article surveillance applications has emerged nor has the prior art recognized the novel approach of this invention.